As a process for producing a glass sheet for various electronic instruments, in particular, a flat panel display such as a liquid crystal display, there is known a down-draw method, in which a glass sheet is produced by causing a molten glass to flow down from a forming trough, and vertically down-drawing a glass ribbon.
The down-draw method includes two methods, i.e., an overflow down-draw method and a slot down-draw method. In particular, the overflow down-draw method is widely known as a method, with which a glass sheet having very small waviness and roughness on its surface and being excellent in surface quality can be obtained.
The overflow down-draw method is a method, which involves continuously feeding a molten glass to a top portion of a forming trough having a wedge-shaped cross-section, causing the molten glass to flow down from the top portion of the forming trough along both side surfaces of the forming trough, and allowing the molten glasses to fuse at a lower end portion of the forming trough to form a sheet-like glass ribbon, and causing the glass ribbon to flow down through a conveyance passage extending vertically while holding the glass ribbon at both edge portions with a plurality of pulling rollers, thereby down-drawing the molten glass into a glass ribbon. With this process, the glass ribbon is gradually solidified, and a glass sheet having a given width and thickness is obtained. Further, atmospheric temperature in the conveyance passage is strictly controlled, and the internal strain (thermal strain) in the glass sheet is sufficiently reduced thereby. Then, the glass sheet is cooled to around room temperature.
In a case of a glass sheet for a liquid crystal display, in particular, if even only a minute internal strain remains in the glass sheet, birefringence occurs, and as a result, a uniform image is not provided. Thus, there have conventionally been proposed various ideas for uniformly cooling a glass ribbon as much as possible at a given temperature gradient.
For example, JP-A-5-124826 discloses a structure including supporting a roller on a single side for preventing occurrence of an internal strain in a glass sheet caused by the influence of the cooling of a roller shaft so as to prevent the deformation of the glass sheet, and also discloses a preventive plate against convection for separating a conveyance passage horizontally for preventing the internal strain in a glass ribbon caused by thermal convection occurring in the conveyance passage.
Further, JP-A-10-53426 discloses a process for producing a glass sheet having a small internal strain, the process involving forming a plurality of chambers by separating an internal space of a forming furnace or an annealing furnace horizontally, and allowing each chamber to have a room temperature-controlling function to carry out sufficient annealing.
Still further, JP-A-2001-31435 discloses a technology by which temperature distribution in the annealing furnace is also formed in a width direction of a glass ribbon, to thereby prevent a minute internal strain or deformation.    Patent Document 1: JP-A-5-124826    Patent Document 2: JP-A-10-53426    Patent Document 3: JP-A-2001-31435